1. Field of the Invention
This invention relates to a heat exchanger apparatus for use with a boiling liquid. More particularly this invention relates to a heat exchanger tube having a fluid to be cooled passing therethrough and a boiling refrigerant in contact with the external surface of the tube.
2. Description of the Prior Art
In certain refrigeration applications such as a chiller or an evaporator, liquid to be cooled is passed through a tube while liquid refrigerant is in contact with the outside of the tube. The refrigerant changes state from a liquid to a vapor, thus absorbing heat from the fluid to be cooled within the tube. The selection of the external configuration of the tube is extremely influential in determining the boiling characteristics and overall heat transfer rate of the tube.
It has been found that the transfer of heat to a boiling liquid is enhanced by the creation of nucleate boiling sites. It has been theorized that the provision of vapor entrapment cavities in the heat exchanger surface creates sites for nucleate boiling.
In nucleate boiling, liquid adjacent to a trapped vapor bubble is superheated by the heat exchanger surface. Heat is transferred to the bubble as this liquid vaporizes at the liquid-vapor interface and the bubble grows in size until surface tension forces are overcome by the buoyancy and momentum forces and the vapor bubble breaks free from the surface. As the bubble leaves the surface, fresh liquid wets the now vacated area and the remaining vapor has a source of additional liquid for creating vapor to form the next bubble The vaporization of liquid and continual stripping of the heated liquid adjacent to the heat transfer surface, together with the convection effect due to the agitation of the liquid pool by the bubbles result in an improved heat transfer rate for the heat exchanger surface. The mechanism for the heat transfer taking place within the vapor entrapment cavities is most accurately described as thin film evaporation.
It is known that the surface heat transfer rate is high in the area where the vapor bubble is formed. Consequently, the overall heat transfer rate tends to increase with the density of vapor entrapment sites per unit area of heat exchanger surface. See for example, U.S. Pat. No. 3,696,861 issued to Webb and entitled "Heat Transfer Surface Having A High Boiling Heat Transfer Coefficient". In the Webb Patent, fins on a heat exchange tube are uni-directionally rolled over toward an adjacent fin to form a narrow gap between adjacent fins. In Webb it is theorized that these narrow gaps create sub surface vapor entrapment sites or cavities and that the narrow gaps act as reentrant openings intercommunicating the entrapment sites or cavities with the boiling liquid.
It is also well known in the theory of boiling heat transfer that tubes having a continuous gap between adjacent fins may suffer from reduced performance in that an excessive influx of liquid refrigerant from the surroundings may be drawn into and flood or deactivate a vapor entrapment site.
The flooding problem has been addressed, and enhanced tubes having sub-surface channels communicating with the surroundings through surface openings or pores which alternate with closed sections have been devised. Such a tubing is shown for example in U.S. Pat. No. 4,438,807 to Mathur et al entitled "High Performance Heat Transfer Tube". The Mathur Patent provides for alternating openings and closed sections wherein the openings for the cavities occur only at those locations above an internal rib or depression formed within the tube.
U.S. Pat. No. 4,765,058, entitled "Apparatus For Manufacturing Enhanced Heat Transfer Surface" issued to the assignee hereof on Aug. 23, 1988 in the name of Zohler. This Patent discloses a finned tube having a plurality of sub-surface channels defined by bent over adjacent fins which communicate with the outside space through a large number of evenly spaced, generally fixed size surface pores.
The '058 Patent points out that the size of the sub-surface channels and the size, number, and configuration of the pores on the surface of the tubes are particularly critical for R-11 applications. It has been found that tubing manufactured according to the teachings of the '058 Patent provide an extremely high performance evaporator tube for use with low pressure refrigerants such as R-11. It has been discovered however that a pore density according to the teachings of the '058 Patent did not produce the expected high performance heat transfer characteristics in higher pressure refrigerants, such as for example, R-22.
R-11 is a member of the family of refrigerants known as Chlorofluorocarbons (CFC's). Recently, there has been a growing scientific consensus that emissions of CFC's are contributing to the depletion of a layer of stratospheric ozone that protects the earth's surface from the harmful effects of ultra violet radiation. International agreements, and, federal and state regulations are being considered that will regulate use, manufacture, importation, and disposal of CFC's in the future R-22 is a member of a chemical family known as hydrochlorofluorocarbons HCFC's). It is believed that because of their hydrogen component, HCFC's break down substantially in the lower atmosphere and, as a result, their ozone depletion potential is substantially lower than that of R-11 and other CFC refrigerants. Accordingly it is expected that R-22 will be used more extensively in the future.